Gallium is now produced mainly from the solutions resulting from the treatment of bauxites following the Bayer method; in doing so, thickening of gallium solutions is performed by various methods.
Known in the art is a process for producing gallium from alkali aluminate solutions resulting from the Bayer process, wherein the concentration is effected by precipitation of aluminum in the form of tricalcium aluminate by means of lime in autoclaves, followed by carbonization of these solutions to convert all the caustic alkali to bicarbonates and to give a concentrate containing 0.3 to 1% by mass of gallium.
This process involves the steps which are not characteristic of the Bayer process; it is accompanied by losses of aluminum and high production costs of the final gallium product.
In another prior art process an alkali aluminate solution resulting from the treatment of bauxites is reacted with carbonic acid to recover about 90% of aluminum in the form of hydroxide thereof; then the solution is stirred and subjected to a repeated carbonization to convert all the caustic alkali to the bicarbonate form thereof. The thus-produced concentrate contains, percent by mass: 0.45 of gallium oxide, 23.6 of carbon dioxide, 47.4 of aluminum oxide, 18.4 of sodium oxide, 9.5 of water. From the resulting concentrate gallium is passed into an alkaline solution, wherefrom gallium is recovered by means of electrolysis. The process does not ensure a required thickening of gallium even from alkali aluminate solutions resulting from the treatment of bauxites containing gallium in greater amounts than alkali aluminate solutions resulting from the treatment of nephelines.
Since the content of gallium in alkali aluminate solutions resulting from the production of alumina from nephelines is by 10 to 20 times as less as in the liquors resulting from the treatment of bauxites by the Bayer process, said methods of concentration and recovery of gallium cannot be successfully applied for the recovery of this metal from intermediate products of the treatment of nephelines.
Known in the art is a process for producing gallium from the solutions resulting from the treatment of aluminum-containing ores which involves stages of concentration and recovery of metallic gallium, wherein concentration of gallium is effected by treatment of a gallium-containing solution with an alloy of mercury with sodium (sodium amalgam) to give a sodium concentrate in mercury containing 0.3 to 3% by mass of gallium, wherefrom gallium is recovered by converting it into an alkaline solution, followed by an electrochemical reduction of gallium, for example on a solid cathode.
Toxic nature of mercury, low solubility of gallium therein, as well as considerable losses of mercury with the liquors being treated -- all this substantially restricts the possibilities of a commercial application of this prior art process.
A common disadvantage of the above-discussed prior art processes for recovering gallium resides in a low degree of concentration of gallium and, consequently, high costs of production of this metal.
It is an object of the present invention to provide such a process which would make it possible to recover gallium from alkali aluminate solutions resulting from the treatment of low-quality aluminum-containing ores such as nephelines at rather low production costs.